The long-term goal of this work is to implement gene-based therapeutics to combat progressive contractile dysfunction in the aging heart. Heart failure is the primary cause of hospitalization, morbidity and mortality in the elderly. Cardiac dystrophin is a key and potentially unifying component of progressive heart dysfunction in aging. Dystrophin is a vital link between the contractile sarcomere and the cell membrane. Dystrophin deficiency is associated with myocardial ischemia and heart failure and causes Duchenne Muscular Dystrophy (DMD), a fatal disease of progressive striated muscle deterioration. DMD is used here as a model of cardiac dysfunction that progressively worsens in aging. New data shows dystrophin protein is decreased in the aging rodent heart. The overarching hypothesis of this proposal is that dystrophin deficiency is causal for cardiac muscle susceptibility to damage in disease and is a central component to increased susceptibility to damage in the aging myocardium. Second hypothesis: increasing cardiac dystrophin protein above baseline levels will confer long-term protection to the myocardium in aged animals in vivo. The Specific Aims are: Aim 1. To determine the aging-dependent effects of intravascular rAAV-mediated micro-dystrophin gene delivery on cardiac performance in young and old wild-type and dystrophin-deficient mice in vivo. Hypotheses: systemic micro-dystrophin gene transfer will prevent aging-dependent ventricular remodeling and confer long-term (year) protection during in vivo cardiac stress testing in vivo; micro- dystrophin will reverse age-dependent ventricular remodeling but not redress maximum pressure deficits; micro-dystrophin will prevent age-dependent decline in diastolic performance in wild-type mice. Aim 2. To optimize mini-dystrophin and hinge modified micro-dystrophin gene cassettes for correction of aging-mediate cardiac hemodynamic deficits in wild-type and dystrophin deficient animals in vivo. Hypothesis: truncated dystrophin with optimized hinge and spectrin-like repeat domains will confer increased cardiac performance relative to first generation micro-dystrophin in dystrophin-deficient mice in vivo. Aim 3. To accomplish age-dependent, full-length dystrophin gene excision/silencing in the hearts of wild- type mice and to directly assess the resulting pathological and hemodynamic outcomes in vivo. Hypothesis: temporal dystrophin gene deletion will cause cardiac injury and heart performance deficits with effects greater in old compared to young animals.